The concentration of water in ambient air may affect engine operation. For example, there may be a 5-8% error in determination of mass air flow in the absence of adjustments based on ambient air humidity. Therefore, engine operating parameters such as air-fuel ratio, spark timing, exhaust gas recirculation (EGR) and the like may be adjusted based on ambient air humidity to improve engine performance, boost fuel economy, and reduce emissions. Further, ambient air humidity may be used to adjust vehicle climate control parameters to improve safety, cabin comfort and driving experience.
Various approaches are utilized to estimate ambient air humidity. In one example approach, as shown by Kim et al. in US 2013/0275030, ambient humidity is measured based on a NOx sensor output. However, inventors herein have recognized disadvantages with such an approach. Specifically, ambient humidity estimated based on NOx sensor output may have reduced accuracy during periods of atmospheric instability such as during conditions when there is precipitation in the atmosphere. Further, when there is a change in humidity, such as during onset of rain, ambient humidity based on a NOx sensor output does not account for the sudden increase in humidity. Still further, the NOx sensor may be utilized to measure humidity only when fuel is shut-off while the engine continues to operate, such as during braking when the vehicle may travel downhill for a short period of time, to allow fresh air to circulate through the engine and the exhaust system. Therefore, it may not be possible to measure humidity when the humidity measurement is needed.
In one example, some of the above issues may be addressed by a method for an engine, comprising: adjusting engine operation based on an ambient specific humidity, the ambient specific humidity estimated based on a dry bulb temperature measured by a first sensor positioned on an exterior surface of a vehicle and shielded from weather, a wet bulb temperature measured by a second sensor positioned on the exterior surface of the vehicle and exposed to weather, and a barometric pressure in response to detecting precipitation.
In another example, a method for an engine comprises indicating a change in a rain condition based on a wet bulb temperature and a dry bulb temperature; and adjusting an estimated humidity based on the change in rain condition, and not utilizing the wet bulb temperature to estimate humidity depending on the rain state.
For example, rain may be detected based on a difference between wet bulb and dry bulb temperatures being greater than a threshold temperature. Upon detecting rain, specific humidity may be estimated based on wet bulb and dry bulb temperatures. Wet bulb temperature may be the temperature of rain drops measured by a wet bulb thermometer located on a surface of the vehicle. Dry bulb temperature may be temperature of intake air measured by a dry bulb thermometer located at an engine intake passage. A psychrometric interpolation table may be used to estimate specific humidity based on wet bulb and dry bulb temperatures. However, during conditions when rain is absent, specific humidity may be estimated based on dry bulb temperature and not wet bulb temperature. In this way, by utilizing wet bulb temperature and dry bulb temperature to estimate ambient specific humidity during rainy conditions, engine operating parameters may be adjusted with greater accuracy. Further, while the term thermometer is used herein as one example temperature sensor, various others may be used such as thermocouples, thermal diode, thermal resistor, etc.
It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.